So, I've only been having this problem intermittently. Recently I changed the oil in my 300DT and overfilled by no more than about 1/2 litre. Since then I have noticed the car doesn't shut off immediately when I turn the key - sometimes it seems to take up to 4 or 5 seconds.

Should I be worried? I might just decide to drain a bit of the excess oil if that's what it comes to. This problem doesn't seem to happen all the time - maybe only one time in three or four. Should I be worried about diesel runaway? My engine has about 307000 km, but the blowby is not at all serious (by the teakettle test).

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1984 300D Turbo, 326000 Km, Thistle Green Metallic

That's characteristic of a vacuum leak. Make sure you ddi not disturb a vacuum line near the oil filter, which is a common occurance.

Also, you might note if there is a difference with the doors unlocked versus locked.

Hmm! Yes, I tried to be extremely careful when pulling off the filter housing cover, but I'll check the vacuum lines tonight when I fiddle with my window switch. It's certainly quite possible that I disturbed something by accident.

I was more concerned that I might be consuming some of my blowby vapours, but that doesn't seem likely - not with being only 500mL over full.

I'll also check shutting off witht he doors locked vs. not.

On a semi-related note, I was just fidgeting yesterday to see how many times i could cycle my locks with the car off and I managed to lock-unlock the car nine times before vacuum was finally insufficient to actuate all the locks. So, at least the vacuum for the locks seems pretty tight!

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1984 300D Turbo, 326000 Km, Thistle Green Metallic

That's a good sign, especially if the engine had been off for a while. In that case, I would focus on the vacuum lines under the hood.